Mood-related disorders such as anxiety and depression are linked to dysfunction in many brain structures, including the hippocampus. Many effective and promising new drugs for the treatment of mood-related disorders target, directly or indirectly, G protein-coupled receptors (GPCRs) that modulate the activity of multiple effectors. The relative significance of specific GPCR-effector interactions to normal and pathological mood-related behaviors, and pertinent anatomic loci, are unclear. The premise of this proposal is that a better understanding of GPCR-effector interactions, obtained with an interdisciplinary approach spanning molecular, ultrastructural, electrophysiological, and behavioral levels of analysis, will ultimately improve how we understand and treat mental disorders with strong affective components. The focus of this study is the G protein-gated inwardly-rectifying K+ (Girk/KIR3) channel, which mediates the postsynaptic inhibitory effect of many neurotransmitters - including several linked mood-related behavior such as GABA, serotonin, and adenosine - that influence neuronal excitability via activation of GPCRs. Recent studies have identified new modes of Girk regulation and a striking GPCR-dependent compartmentalization of Girk signaling in the hippocampus. The goal of the proposed research is to build on this foundation by pursuing factors influencing the strength and sensitivity of GPCR-Girk signaling, mechanisms underlying the GPCR-dependent compartmentalization of Girk signaling in the hippocampus, and the contributions made by hippocampal GPCR-Girk signaling to mood-related behavior and cognition. Effort will be centered on three inter-related Specific Aims: (1) To understand the Girk1-dependent potentiation of GPCR-Girk signaling. Structural insights into the positive contribution made by the Girk1 subunit to GPCR-Girk signaling will be pursued, and the hypothesis that Girk1 strengthens the physical interaction between receptor and channel will be tested. (2) To identify factors underlying the GPCR-dependent compartmentalization of Girk signaling. The hypothesis that the molecular composition and subcellular distribution of hippocampal Girk signaling differs in a GPCR- dependent manner will be tested. (3) To measure the impact of hippocampal GPCR-Girk signaling on mood and cognition. Mood-related behavior and cognition will be measured in constitutive knockout mice exhibiting diminished or enhanced GPCR signaling, and in mice following pharmacologic and/or genetic manipulation of Girk signaling in the hippocampus. The molecular, subcellular, and behavioral insights gleaned from these studies will enhance our understanding of neuronal GPCR-Girk signaling, while offering new insights into the molecular and anatomic basis of mood-related behavior and cognition. Accordingly, the proposed research aligns with several high-priority research areas at the National Institute of Mental Health and constitutes a necessary step toward a more comprehensive understanding of, and more effective treatments for, mood- related disorders.